repositoryformatversion and extensions

[Edward Thomson <ethomson@edwardthomson.com>]

While looking at worktree configuration, I realized that the
extensions.worktreeconfig configuration option was honored even when
core.repositoryformatversion was unset, or explicitly set to 0.

Re-reading the docs for repository-version, it doesn't explicitly state
that extensions _require_ repositoryformatversion >= 1, it instead
states that:

> When reading the core.repositoryformatversion variable, a git
> implementation which supports version 1 MUST also read any configuration
> keys found in the extensions section of the configuration file.

Despite that, at least one extension (objectformat) does require
core.repositoryformatversion >= 1. Though, do note that objectformat
is not mentioned in the repository-version documentation.

What other extensions require repository format version >= 1? Is
core.objectformat the outlier here?

Is there documentation on which extensions are supported at which
repository format versions?

Are there other extensions that are not documented in repository-version?

Cheers-
Ed